Planarization is increasingly important in semiconductor manufacturing techniques. As device sizes decrease, the importance of achieving high resolution features through photolithographic processes correspondingly increases thereby placing more severe constraints on the degree of planarity required of a semiconductor wafer processing surface. Excessive degrees of surface non-planarity will undesirably affect the quality of several semiconductor manufacturing process including, for example, photolithographic patterning processes, where the positioning the image plane of the process surface within an increasingly limited depth of focus window is required to achieve high resolution semiconductor feature patterns.
A method of choice for planarizing a semiconductor process wafer following a particular microelectronic circuit fabrication semiconductor wafer manufacturing process such as deposition of a material layer is chemical mechanical polishing (CMP), which can achieve sufficient degrees of both local and global planarization in modern manufacturing processes necessary for successfully carrying out subsequent feature forming processes, for example photolithographic processes which require a high degree of planarization to maintain feature design critical dimensions. The planarity of CMP processes is increasingly critical especially for devices having narrow semiconductor features such as line widths below about 0.25 microns and in the future less than 0.1 micron. CMP planarization is typically used several different times in the manufacture of a multi-level semiconductor device, including planarizing levels of a device containing overlying layers of silicon oxide to achieve local and global planarization for subsequent processing of overlying levels.
During a typical CMP process, the process wafer is mounted on a CMP platen with the process surface face down in contact with a polishing pad onto which polishing slurry is periodically deposited. A down force is applied to the wafer and the wafer process surface is placed in motion with while contacting the polishing pad surface which is also typically placed in motion. The CMP process typically requires polishing slurry that is selective with respect to an underlying material layer to prevent over polishing to include polishing the underlying layer which may detrimentally affect the planarity of the CMP process resulting in erosion and/or dishing. Further, over polishing may result in scratching of surface features where over polishing of the underlying layer is formed of a harder material.
Since slurries are frequently insufficiently selective to prevent such over polishing, CMP processes typically employ an endpoint detection system to determine a point at which the underlying layer is exposed. Endpoint detection systems have been proposed that are both in-situ and in-line. For example, optical systems relying on reflected visible or UV light have been employed in-situ during the polishing process where light is directed at a predetermine angle at the wafer process surface and reflected light is detected and analyzed to determine whether an underlying layer has been exposed. Other in-situ systems have relied on detection of chemicals in the polishing effluent or the change in polishing friction reflected in the amount of electrical current drawn by motors used to move either the polishing platen or the polishing pad surface. In-situ optical detection systems frequently suffer from adequate focusing or inadequate signal intensity to make accurate endpoint detection determinations and are frequently unworkable where no polishing breakthrough from one material layer to another occurs. In-line systems have been developed to overcome theses shortcomings where the process wafer is moved to a nearby wafer thickness measuring station where the wafer process surface is probed to determine whether polishing breakthrough has occurred. For example, such measurements have typically relied on UV or visible light reflectivity or optical interferometry methods.
A shortcoming of endpoint detection systems of the prior art is that the detecting methodology typically relies on the occurrence of a polishing breakthrough with respect to the targeted polishing layer to expose a materially different underlying layer. Frequently, especially in the case of oxide polishing where only a portion of the oxide is polished, no polishing breakthrough occurs, frustrating many of the techniques of the prior art. Further, in cases where a polishing breakthrough occurs thus indicating the exposure of an underlying layer, prior art techniques do not adequately indicate the amount of remaining oxide or allow local probing of the process surface to determine whether substantially the entire overlying oxide layer is removed. As a result, in-line endpoint detection systems of the prior art do not allow updated projections to be made with respect to a projected remaining polishing time or a determination that substantially all of the oxide from the target polishing layer has been removed.
Therefore, there is a need in the semiconductor wafer microelectronic integrated circuit processing art for an improved method for measuring the thickness of an oxide layer for example, including determining the remaining thickness of an oxide layer in a CMP process to improve endpoint detection and to ensure substantially complete removal of the oxide layer.
It is therefore an object of the invention to provide an improved method for measuring the thickness of an oxide layer for example, including determining the remaining thickness of an oxide layer in a CMP process to improve endpoint detection and to ensure substantially complete removal of the oxide layer while overcoming other shortcomings and deficiencies in the prior art.